Ion sensitive electrodes are generally utilized in the medical field to detect ion levels in fluids. Typically, these electrodes detect ion levels by measuring the electrical potential (EMF--electromotive force) generated across a membrane separating two solutions with different concentrations of ions. Alternatively, a nonporous, electrically non-conductive film, such as polyvinyl chloride, impregnated with an ionophore specific to the ion to be measured can be employed. In the absence of the ionophore, the film is an insulator and no EMF can be measured across the film, When the film is blended with an ionophore, charged ions are bound to the film and a small, measurable current can be induced to flow. Because the ionophore is selective in its affinity, and thus will bind only certain specific ions, such electrodes are ion selective. Any measurable EMF is due solely to the presence of the bound ions.
Generally, ion sensitive electrodes are used in conjunction with an analyzer instrument as part of a diagnostic system. Examples of disposable ion sensitive electrode sensors are disclosed in U.S. patent application Ser. No. 07/745,971 entitled "Method and Apparatus for Single Determination Blood Analysis", and U.S. patent application Ser. No. 07/220,246 entitled "Precalibrated, Disposable Electrochemical Sensors", now abandoned. The disclosure of each of these patent applications is incorporated herein by reference.
The system may contain a sensor portion, that may be disposable, that places a fluid, such as blood, in contact with the ion sensitive electrode. After the fluid has been placed in contact with the ion sensitive electrode, the analyzer instrument portion of the system, containing electrical circuitry, analyzes the ion level of the fluid. For example, in the case of a blood sample, the potassium ion level may be measured.
Diagnostic systems wherein the sensor portion and the analyzer instrument are part of a single unit are generally known. Systems having a disposable (single use) sensor portion adapted to communicate with a non-disposable analyzer instrument are also generally known. Existing design of disposable, single use ion selective electrode sensors calls for the disposable potentiometric sensing elements to be mounted on an electrically non-conductive support that is inserted into the re-usable analyzer instrument for analysis of the potentiometric electronic signals and thereby the ion level of the sample. The electrical signals transmitted between disposable sensor and the analyzer instrument are characteristically of low current and low direct voltage and are therefore susceptible to electronic noise and to poor electrical contacts in the measurement circuit.
Additionally, in generally known systems, in order to ensure good electrical contact between the conductive elements of the sensor and the analyzer instrument, a sliding movement is utilized during the insertion of the disposable sensor portion into the analyzer instrument. This sliding movement, in generally known systems, causes the non-conductive support of the sensor portion to slide against a non-conductive part of the analyzing instrument. This relative movement of the two non-conductive parts generally causes significant errors in the potentiometric measurement due to the generation of static electrical charges and associated electrical fields.
Further, in generally known systems, the sensor portion is exposed, at least in part, after it has been connected to the analyzer instrument. Thus, the ion selective electrode, and the contact electrodes between the sensor portion and analyzer instrument are at least partially exposed to external electrical fields. These external electrical fields may cause errors in the measurement of the ion level of the fluid sample.
The foregoing disadvantages of heretofore generally known systems are overcome, and additional advantages achieved by the system of the present invention.